Light receiving device

ABSTRACT

A light receiving element includes a core configured to propagate a signal light, a first semiconductor layer having a first conductivity type, the first semiconductor layer being configured to receive the signal light from the core along a first direction in which the core extends, an absorbing layer configured to absorb the signal light received by the first semiconductor layer, and a second semiconductor layer having a second conductivity type opposite to the first conductivity type.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2011-061842, filed on Mar. 20,2011, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a light receivingelement, a light receiving device, and a light receiving module.

BACKGROUND

FIG. 1 is a perspective view illustrating the main portion of a lightreceiving element 100 as an example of light receiving elements. FIG. 2is a cross-sectional view of the light receiving element 100 taken alonga broken line II-II in FIG. 1.

The light receiving element 100 illustrated as FIGS. 1 and 2 includes aphoto-detector unit 101 provided over a substrate 114, and a waveguideunit 111 provided over the same substrate 114. The waveguide unit 111includes a core 112 having a rib-like shape. A signal light propagateswithin a projection 113 of the core 112, and enters the photo-detectorunit 101.

The photo-detector unit 101 has a structure in which the core 112, ann-type semiconductor layer 102, an i-type absorbing layer 103, a p-typeupper clad layer 104, and a p-type contact layer 105 are laminated fromthe substrate 114 side. The photo-detector unit 101 has a mesa structureincluding the p-type contact layer 105, the p-type upper clad layer 104,and the i-type absorbing layer 103. The core 112 exists both in thephoto-detector unit 101 and the waveguide unit 111.

As illustrated as FIG. 2, in the light receiving element 100, the signallight propagates below the projection 113 of the core 112 in thewaveguide unit 111, and enters the core 112 in the photo-detector unit101. Part of the incident signal light seeps out into the n-typesemiconductor layer 102. As the signal light further propagates in thephoto-detector unit 101, the signal light spreads to the i-typeabsorbing layer 103, and is absorbed in the i-type absorbing layer 103.

The n-type semiconductor layer 102, the i-type absorbing layer 103, andthe p-type upper clad layer 104 form a PIN-type photo diode (hereinafterreferred to as PD). A p-side electrode and an n-side electrode (notillustrated) are connected to the p-type contact layer 105 and then-type semiconductor layer 102, respectively. By applying apredetermined voltage between the p-side electrode and the n-sideelectrode, with the p-side electrode at a negative potential and then-side electrode at a positive potential, photocarriers (holes andelectrons) generated by light absorption in the i-type absorbing layer103 are detected via the p-type upper clad layer 104 and the n-typesemiconductor layer 102. Thus, the photo-detector unit 101 detects thesignal light as an electrical signal (photocarrier current), and outputsa detection signal (photocarrier current) corresponding to the intensityof the signal light.

FIG. 3 illustrates the main portion of a light receiving element 300 asanother example of light receiving element. FIG. 4 is a cross-sectionalview of the light receiving element 300 taken along a broken line IV-IVin FIG. 3.

The light receiving element 300 illustrated as FIGS. 3 and 4 has astructure different from that of the light receiving element 100illustrated as FIGS. 1 and 2. The light receiving element 300 includes aphoto-detector unit 301 provided over a substrate 314, and a waveguideunit 311 provided over the same substrate 314.

The waveguide unit 311 has a structure in which an n-type lower cladlayer 302, a core 312, and an upper clad layer 313 are laminated fromthe substrate 314 side. The waveguide unit 311 has a mesa structureincluding the upper clad layer 313 and the core 312. A signal lightpropagates in the core 312, and enters the photo-detector unit 301.

The photo-detector unit 301 has a structure in which the n-type lowerclad layer 302, an i-type absorbing layer 303, a p-type upper clad layer304, and a p-type contact layer 305 are laminated from the substrate 314side. The photo-detector unit 301 has a mesa structure including thep-type contact layer 305, the p-type upper clad layer 304, and thei-type absorbing layer 303.

The core 312 and the i-type absorbing layer 303 are both formed on then-type lower clad layer 302 that is shared by the photo-detector unit301 and the waveguide unit 311. The core 312 is connected to a side faceof the i-type absorbing layer 303.

As illustrated as FIG. 4, in the light receiving element 300, the signallight propagates in the core 312 in the waveguide unit 311, and directlyenters the i-type absorbing layer 303 in the photo-detector unit 301.The incident signal light is absorbed in a region near the end of thei-type absorbing layer 303 from which the signal light enters.

The n-type lower clad layer 302, the i-type absorbing layer 303, and thep-type upper clad layer 304 form a PIN-type photodiode. A p-sideelectrode and an n-side electrode are connected to the p-type contactlayer 305 and the n-type lower clad layer 302, respectively. By applyinga predetermined voltage between the p-side electrode and the n-sideelectrode, with the p-side electrode at a negative potential and then-side electrode at a positive potential, photocarriers (holes andelectrons) generated by light absorption in the i-type absorbing layer303 are detected via the p-type upper clad layer 304 and the n-typelower clad layer 302. Thus, the photo-detector unit 301 detects thesignal light as an electrical signal (photocarrier current), and outputsa detection signal (photocarrier current) corresponding to the intensityof the signal light.

An example of the two light receiving elements 100 and 300 illustratedas FIGS. 1 to 4 is discussed in Japanese Laid-open Patent PublicationNo. 2003-163363 and Andreas Beling et al. J. Lightwave Tech., VOL. 27,NO. 3, pp 343-355, Feb. 1, 2009.

FIG. 5 illustrates an example of simulated density distributions ofphotocarriers generated in the photo-detector units 101 and 301. Thevertical axis represents the density of photocarriers as normalized onthe basis of a predetermined value. The horizontal axis representslocation inside the PD in each of the photo-detector units 101 and 301.In this specification, the term “location inside the PD” refers to alocation inside the PD along the direction in which the correspondingcore extends, that is, the direction of travel of the signal light, andmeans a location inside the PD included in the photo-detector unit withreference to the end from which the signal light enters. Also, the term“PD length” refers to the length of the PD in the photo-detector unitalong the direction in which the corresponding core extends, that is,the direction of travel of the signal light, and means the length of thePD in the photo-detector unit with reference to the end from which thesignal light enters.

In FIG. 5, the curve indicated by (a) represents photocarrier densitydistribution in the photo-detector unit 101 illustrated as FIGS. 1 and2, and the curve indicated by (b) represents photocarrier densitydistribution in the photo-detector unit 301 illustrated as FIGS. 3 and4.

In the photo-detector unit 101 of the light receiving element 100, thesignal light seeps out into the i-type absorbing layer 103 afterpropagating through the core 112 and the n-type semiconductor layer 102in the photo-detector unit 101 by a predetermined distance, andabsorption thus takes place. Accordingly, as is apparent from thedistribution curve (a) in FIG. 5, the peak of the photocarrier densitydistribution occurs at a location separated by a predetermined distancefrom the end of the photo-detector unit 101 from which the signal lightenters. Moreover, the overall density distribution also spreads to alocation farther away from the end of the photo-detector unit 101, sothe distribution has a large spread as a whole.

Therefore, in the photo-detector unit 101, the PD length of thephoto-detector unit 101 is set to a sufficient length for obtainingsufficient absorption efficiency. However, making the PD length of thephoto-detector unit 101 longer increases the size of the capacitorincluding the n-type semiconductor layer 102, the i-type absorbing layer103, and the p-type upper clad layer 104, causing an increase incapacitance of the photo-detector unit 101. Thus, the cut-off frequencyderived from the CR time constant becomes lower in the transmission pathbetween the light receiving element 100 and the subsequent electricalcircuit. Consequently, in the subsequent electrical circuit thatreceives an electrical signal outputted from the light receiving element100, the level of the input signal attenuates at high frequencies,making it difficult to appropriately process the input signal also athigh frequencies.

As a result, with the structure of the light receiving element 100illustrated as FIGS. 1 and 2, it is difficult to supply a detectionsignal with sufficient signal level to the subsequent electrical circuitwhile ensuring high light absorption efficiency.

On the other hand, in the photo-detector unit 301 of the light receivingelement 300, the signal light having propagated through the core 312directly enters the i-type absorbing layer 303. Consequently, largeabsorption takes place in the vicinity of the end of the photo-detectorunit 301. Thus, as is apparent from the distribution curve (b) in FIG.5, photocarriers generate and their density becomes high within a narrowrange near the end of the photo-detector unit 301 from which the signallight enters.

Therefore, in the photo-detector unit 301, high light absorptionefficiency is obtained even if the PD length is short. However, sincethe rising edge of the photocarrier density distribution is so largenear the end of the photo-detector unit 301 that in the case of highintensity light input where the intensity of the inputted signal lightis high, the density of photocarriers locally generated near the end ofthe photo-detector unit 301 becomes excessively high. As a result, inthe photo-detector unit 301, a large electric field is created betweenthe p-type upper clad layer 304 and the n-type lower clad layer 302 bythe excessive locally generated photocarriers, in a direction oppositeto the electric field created by the above-mentioned voltage appliedbetween the p-side electrode and the n-side electrode. The electricfield created by the excessive locally generated photocarriers acts tocancel out the electrical field created by the above-mentioned voltageapplied between the p-side electrode and the n-side electrode. Thismakes it difficult for the photo-detector unit 301 to appropriatelydetect the photocarriers (holes and electrons) generated by lightabsorption in the i-type absorbing layer 303 via the p-type upper cladlayer 304 and the n-type lower clad layer 302. Consequently, the highfrequency property for high intensity signal light deteriorates.

As a result, with the structure of the light receiving element 300illustrated as FIGS. 3 and 4, it is difficult to perform an outputoperation adapted to high intensity light input.

SUMMARY

According to an aspect of the invention, a light receiving elementincludes a core configured to propagate a signal light, a firstsemiconductor layer having a first conductivity type, the firstsemiconductor layer being configured to receive the signal light fromthe core along a first direction, the core extending in the firstdirection, an absorbing layer configured to absorb the signal lightreceived by the first semiconductor layer, and a second semiconductorlayer having a second conductivity type opposite to the firstconductivity type.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating an example of a lightreceiving element;

FIG. 2 is a cross-sectional view of a light receiving element takenalong a broken line II-II in FIG. 1;

FIG. 3 is a perspective view illustrating another example of a lightreceiving element;

FIG. 4 is a cross-sectional view of a light receiving element takenalong a broken line IV-IV in FIG. 3;

FIG. 5 illustrates an example of simulated light absorptiondistributions in photo-detector units;

FIG. 6 is a perspective view illustrating an example of the structure ofa light receiving element according to a first embodiment;

FIG. 7 is a cross-sectional view of a light receiving element takenalong a broken line VII-VII in FIG. 6;

FIG. 8 illustrates an example of the simulated density distribution ofphotocarriers generated in a photo-detector unit of a light receivingelement;

FIG. 9 illustrates an example of simulated quantum efficiency in aphoto-detector unit of a light receiving element;

FIG. 10 is a perspective view illustrating an example of the structureof a light receiving element;

FIG. 11 is a cross-sectional view of a light receiving element takenalong a broken line XI-XI in FIG. 10;

FIGS. 12A and 12B are cross-sectional views of a light receiving elementtaken along broken lines XIIA-XIIA and XIIB-XIIB in FIG. 10,respectively;

FIGS. 13A and 13B illustrate an example of the manufacturing process ofthe light receiving element illustrated as FIGS. 10 to 12B (Part 1);

FIGS. 14A and 14B illustrate an example of the manufacturing process ofthe light receiving element illustrated as FIGS. 10 to 12B (Part 2);

FIGS. 15A and 15B illustrate an example of the manufacturing process ofthe light receiving element illustrated as FIGS. 10 to 12B (Part 3);

FIGS. 16A and 16B illustrate an example of the manufacturing process ofthe light receiving element illustrated as FIGS. 10 to 12B (Part 4);

FIGS. 17A and 17B illustrate an example of the manufacturing process ofthe light receiving element illustrated as FIGS. 10 to 12B (Part 5);

FIG. 18 is a cross-sectional view illustrating an example of thestructure of a light receiving element according to a second embodiment;

FIG. 19 is a cross-sectional view illustrating an example of thestructure of a light receiving element according to a third embodiment;

FIG. 20 illustrates an example of the simulated density distribution ofphotocarriers generated in a photo-detector unit of a light receivingelement;

FIG. 21 is a cross-sectional view illustrating an example of thestructure of a light receiving element according to a fourth embodiment;

FIG. 22 is a plan view illustrating an example of the configuration of alight receiving device according to a fifth embodiment; and

FIG. 23 is a plan view illustrating an example of the configuration of alight receiving module according to a sixth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments are described.

Embodiments 1. First Embodiment 1-1. Structure of a Light ReceivingElement 600

FIG. 6 is a perspective view illustrating an example of the structure ofa light receiving element 600 according to a first embodiment. FIG. 6illustrates only the main portion of the light receiving element 600.FIG. 7 is a cross-sectional view of the light receiving element 600taken along a broken line VII-VII in FIG. 6. FIG. 7 illustrates thevicinity of the boundary region between a photo-detector unit 601 and awaveguide unit 611.

In this specification, with respect to the side of the substrate surfaceon which the structure of a light receiving element is formed, thedirection pointing away from the substrate surface is referred to as“upper”, “top”, “over”, “on” or “above”, and the direction pointingtowards the substrate surface is referred to as “lower”, “bottom”,“under”, or “below”.

As illustrated as FIGS. 6 and 7, the light receiving element 600includes the photo-detector unit 601 provided over a substrate 614, andthe waveguide unit 611 provided over the same substrate 614.

The waveguide unit 611 has a structure in which a core 612 and an upperclad layer 613 are laminated from the substrate 614 side. The materialof each of the layers of this laminate structure is, for example, asemiconductor. The waveguide unit 611 has a mesa structure including theupper clad layer 613 and the core 612. A signal light propagates in thecore 612, and enter the photo-detector unit 601.

The photo-detector unit 601 has a structure in which an n-typesemiconductor layer 602, an i-type absorbing layer 603, a p-type upperclad layer 604, and a p-type contact layer 605 are laminated from thesubstrate 614 side. The material of each of the layers of this laminatestructure is, for example, a semiconductor. The photo-detector unit 601has a mesa structure including the p-type contact layer 605, the p-typeupper clad layer 604, the i-type absorbing layer 603, and part of then-type semiconductor layer 602. The n-type semiconductor layer 602, thei-type absorbing layer 603, and the p-type upper clad layer 604 form aPIN-type photodiode.

As illustrated as FIG. 7, in the light receiving element 600, the core612 is connected to a side face of the n-type semiconductor layer 602,and the core 612 and the n-type semiconductor layer 602 are connectedadjacent to each other along the direction in which the core 612extends. The core 612 is so formed that its top face is located higherthan the bottom face of the n-type semiconductor layer 602 (locatedfarther away from the substrate 614), and its bottom face is locatedlower than the top face of the n-type semiconductor layer 602 (locatedcloser to the substrate 614).

The n-type semiconductor layer 602 is so formed that its refractiveindex is higher than the refractive index of the core 612, and lowerthan the refractive index of the i-type absorbing layer 603. That is,the n-type semiconductor layer 602 is so formed that its bandgapwavelength is longer than the bandgap wavelength of the core 612, andshorter than the bandgap wavelength of the i-type absorbing layer 603.The n-type semiconductor layer 602 is formed so as to have a compositionthat makes the absorption coefficient for the signal light sufficientlysmall.

As illustrated as FIG. 7, in the light receiving element 600, the signallight propagates in the core 612 in the waveguide unit 611, and entersthe n-type semiconductor layer 602 in the photo-detector unit 601. Then-type semiconductor layer 602 receives the signal light from the core612 along the direction in which the core 612 extends. Since therefractive index of the n-type semiconductor layer 602 is set higherthan the refractive index of the core 612, the loss at the time when thesignal light enters the n-type semiconductor layer 602 from the core 612can be reduced. Part of the incident signal light seeps out from then-type semiconductor layer 602 into the i-type absorbing layer 603, andis absorbed in the i-type absorbing layer 603.

A p-side electrode and an n-side electrode (not illustrated) areconnected to the p-type contact layer 605 and the n-type semiconductorlayer 602, respectively. By applying a predetermined voltage between thep-side electrode and the n-side electrode, with the p-side electrode ata negative potential and the n-side electrode at a positive potential,photocarriers (holes and electrons) generated by light absorption in thei-type absorbing layer 603 are detected via the p-type upper clad layer604 and the n-type semiconductor layer 602. Thus, the photo-detectorunit 601 detects the signal light as an electrical signal, and generatesthe detection signal as an electrical signal (photocarrier current). Thephoto-detector unit 601 outputs the detection signal (photocarriercurrent) corresponding to the intensity of the signal light to thesubsequent electrical circuit.

In the photo-detector unit 601, unlike in the photo-detector unit 101 ofthe light receiving element 100 illustrated as FIGS. 1 and 2, the signallight enters the n-type semiconductor layer 602 which is directly incontact with the i-type absorbing layer 603 in which absorption of thesignal light takes place. Consequently, in the photo-detector unit 601,as soon as the signal light enters the n-type semiconductor layer 602,the signal light seeps out into the i-type absorbing layer 603, so thatabsorption of the signal light begins immediately after the entry.Consequently, in the photo-detector unit 601, the PD length can be madeshorter than the PD length of the photo-detector unit 101 while ensuringsufficient light absorption efficiency.

Because the PD length can be shortened in the photo-detector unit 601,the size of the capacitor including the n-type semiconductor layer 602,the i-type absorbing layer 603, and the p-type upper clad layer 604 canbe made smaller. Consequently, capacitance of the photo-detector unit601 can be reduced, and thus the cut-off frequency derived from the CRtime constant becomes higher in the transmission path between the lightreceiving element 600 and the subsequent electrical circuit.Consequently, the light receiving element 600 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at high frequencies, making it possible for the subsequentelectrical circuit to process an input signal also at high frequencies.

In addition, in the light receiving element 600, unlike in thephoto-detector unit 301 of the light receiving element 300 illustratedas FIGS. 3 and 4, the signal light does not directly enter the i-typeabsorbing layer 603 but enters the n-type semiconductor layer 602 first,and then its components seeping out into the i-type absorbing layer 603are absorbed. Consequently, in the photo-detector unit 601, the overalldensity distribution of generated photocarriers is a flat, gentlysloping distribution in comparison to that in the photo-detector unit301.

Consequently, in the photo-detector unit 601, an excessive increase inlocal photocarrier density can be reduced even in the case of highintensity light input where the intensity of the inputted signal lightis high. Thus, in the photo-detector unit 601, it is possible to reducedeterioration of high frequency property for high intensity signal lightdue to the influence of an electric field created by photocarriers(holes and electrons). Therefore, in the light receiving element 600, itis possible to perform an output operation adapted to high intensitylight input.

As has been described above, in the light receiving element 600, adetection signal with sufficient signal level can be supplied to thesubsequent electrical circuit also at high frequencies while improvingthe efficiency of light absorption in the photo-detector unit 601.Further, the light receiving element 600 can perform an output operationadapted to high intensity light input in which the intensity of theinput signal light is high.

1-2. Photocarrier Density Distribution in the Photo-Detector Unit 601

FIG. 8 illustrates an example of the simulated density distribution ofphotocarriers generated in the photo-detector unit 601 of the lightreceiving element 600. The vertical axis represents the density ofphotocarriers as normalized on the basis of a predetermined value. Thehorizontal axis represents location inside the PD in the photo-detectorunit 601.

In FIG. 8, the curve indicated by (c) represents photocarrier densitydistribution in the photo-detector unit 601 of the light receivingelement 600 illustrated as FIGS. 6 and 7. In FIG. 8, for the sake ofcomparison, photocarrier density distribution in the photo-detector unit101 of the light receiving element 100 illustrated as FIGS. 1 and 2 isrepresented as the curve (a), and photocarrier density distribution inthe photo-detector unit 301 of the light receiving element 300illustrated as FIGS. 3 and 4 is represented as the curve (b). Thedistribution curves (a) and (b) in FIG. 8 are the same as thedistribution curves (a) and (b) in FIG. 5, respectively.

As represented as the distribution curve (c), in the case of the lightreceiving element 600, as compared with the light receiving element 100(distribution curve (a)), the peak of the photocarrier densitydistribution is located closer to the end of the photo-detector unit601. Accordingly, the overall photocarrier density distribution is alsoshifted to the side closer to the end of the photo-detector unit 601,and the spread of the distribution as a whole is also small.

Therefore, in the light receiving element 600, the PD length of thephoto-detector unit 601 can be made shorter than the PD length of thephoto-detector unit 101 in the light receiving element 100 whileensuring sufficient light absorption efficiency. For example, it isestimated from the amount of shift of the peak location that, in orderto obtain the same light absorption coefficient, about ⅔ of the PDlength of the photo-detector unit 101 is sufficient as the PD length ofthe photo-detector unit 601.

Because the PD length can be shortened, capacitance of thephoto-detector unit 601 also becomes smaller than the capacitance of thephoto-detector unit 101. Thus, the light receiving element 600 cansupply a detection signal with sufficient signal level to the subsequentelectrical circuit also at high frequencies, making it possible for thesubsequent electrical circuit to process an input signal also at highfrequencies.

In addition, as represented as the distribution curve (c), in the lightreceiving element 600, the peak of the photocarrier density distributionis sufficiently low in comparison to that in the light receiving element300 (distribution curve (b)). In the light receiving element 600, thevalue of density at the peak location of the photocarrier densitydistribution is sufficiently small in comparison to the maximum value ofdensity in the case of the light receiving element 300 (distributioncurve (b)). Also, the density distribution as a whole is a flat, gentlysloping distribution as compared with that of the light receivingelement 300 (distribution curve (b)).

Therefore, an excessive increase in local photocarrier density can bereduced even in the case of high intensity light input where theintensity of the inputted signal light is high. Thus, in the lightreceiving element 600, it is possible to reduce deterioration of highfrequency property for high intensity signal light.

As has been described above, in the light receiving element 600, adetection signal with sufficient signal level can be supplied to thesubsequent electrical circuit also at high frequencies while improvingthe efficiency of light absorption in the photo-detector unit 601.Further, the light receiving element 600 can perform an output operationadapted to high intensity light input in which the intensity of theinput signal light is high.

1-3. Quantum Efficiency in the Photo-Detector Unit 601

FIG. 9 illustrates an example of simulated quantum efficiency in thephoto-detector unit 601 of the light receiving element 600. The verticalaxis represents quantum efficiency. The horizontal axis represents thePD length of the photo-detector unit 601.

In FIG. 9, the curve indicated by (a) represents quantum efficiency inthe photo-detector unit 601 of the light receiving element 600illustrated as FIGS. 6 and 7. In FIG. 9, for the sake of comparison,quantum efficiency in the photo-detector unit 101 of the light receivingelement 100 illustrated as FIGS. 1 and 2 are represented as the curve(b).

As is apparent from FIG. 9, the quantum efficiency of the lightreceiving element 600 is generally higher than the quantum efficiency ofthe light receiving element 100. That is, the light receiving element600 can improve quantum efficiency by adopting the structure illustratedas FIGS. 6 and 7 in comparison to the light receiving element 100. Forexample, to obtain a quantum efficiency of 80%, a length that is about60% of the PD length of the light receiving element 100 is sufficient asthe PD length of the light receiving element 600. Also, for example,with the same PD length of 10 μm, the light receiving element 600 canprovide a quantum efficiency about 1.5 times higher than that of thelight receiving element 100.

Therefore, in the light receiving element 600, the efficiency of lightabsorption in the photo-detector unit 601 can be improved.

1-4. Specific Example of the Structure of the Light Receiving Element600

FIG. 10 is a perspective view illustrating an example of the structureof a light receiving element 1000. FIG. 10 illustrates only the mainportion of the light receiving element 1000. The structure of the lightreceiving element 1000 illustrated as FIG. 10 is a specific example ofthe structure of the light receiving element 600 illustrated as FIG. 6,and specifically illustrates a configuration example of each layer ofthe light receiving element 600. FIG. 11 is a cross-sectional view ofthe light receiving element 1000 taken along a broken line XI-XI in FIG.10. FIG. 11 illustrates the vicinity of the boundary region between aphoto-detector unit 1001 and a waveguide unit 1011. FIG. 12A is across-sectional view of the light receiving element 1000 taken along abroken line XIIA-XIIA in FIG. 10, illustrating the vicinity of a mesastructure. FIG. 12B is a cross-sectional view of the light receivingelement 1000 taken along a broken line XIIB-XIIB in FIG. 10,illustrating the vicinity of a mesa structure.

As illustrated as FIGS. 10 to 12B, the light receiving element 1000includes, for example, the photo-detector unit 1000 provided over ansemi-insulating (hereinafter referred to as SI) InP substrate 1014 andthe waveguide unit 1011 provided over the same SI-InP substrate 1014. Anelement that forms a deep impurity level such as Fe is doped in theSI-InP substrate 1014.

The waveguide unit 1011 has a structure in which an i-InGaAsP core layer1012 made of i-type InGaAsP having a bandgap wavelength of 1.05 μm, andan i-InP clad layer 1013 made of i-type InP are laminated from theSI-InP substrate 1014 side. The waveguide unit 1011 has a mesa structureincluding the i-InP clad layer 1013 and the i-InGaAsP core layer 1012,and has a high-mesa waveguide structure whose side faces are not buriedwith a semiconductor material.

The photo-detector unit 1001 has a structure in which an n-InGaAsPsemiconductor layer 1002 made of n-type InGaAsP having a bandgapwavelength of 1.3 μm, an i-InGaAs absorbing layer 1003 made of i-typeInGaAs lattice-matched with InP, a p-InP clad layer 1004 made of p-typeInP, and a p-type contact layer 1005 made up of a two-layer structure ofp-type InGaAs and InGaAsP are laminated from the SI-InP substrate 1014side.

The photo-detector unit 1001 has a mesa structure including the p-typecontact layer 1005, the p-InP clad layer 1004, the i-InGaAs absorbinglayer 1003, and part of the n-InGaAsP semiconductor layer 1002. Thephoto-detector unit 1001 has a high-mesa waveguide structure whose sidefaces are not buried with a semiconductor material. The n-InGaAsPsemiconductor layer 1002, the i-InGaAs absorbing layer 1003, and thep-InP clad layer 1004 form a PIN-type photodiode.

A p-side electrode 1015 is formed over the p-type contact layer 1005,and an n-side electrode 1016 is formed over the n-InGaAsP semiconductorlayer 1002. The portion of the light receiving element 1000 where thep-side electrode 1015 and the n-type electrode 1016 are not formed iscovered with a passivation film 1017 made of a dielectric such as asilicon nitride film. In FIG. 10, the passivation film 1017 is notillustrated to facilitate understanding of the structure.

A predetermined voltage with the p-side electrode 1015 at a negativepotential and the n-side electrode 1016 at a positive potential isapplied between the p-side electrode 1015 and the n-side electrode 1016.Thus, photocarriers (holes and electrons) generated by light absorptionin the i-InGaAs absorbing layer 1003 are detected via the p-InP cladlayer 1004 and the n-InGaAsP semiconductor layer 1002.

In the waveguide unit 1011, for example, the thickness of the i-InGaAsPcore layer 1012 is set to 0.5 μm, and the thickness of the i-InP cladlayer 1013 is set to 1.5 μm. In the photo-detector unit 1001, forexample, the thickness of the n-InGaAsP semiconductor layer 1002 is setto 0.5 μm, the thickness of the i-InGaAs absorbing layer 1003 is set to0.5 μm, and the total thickness of the p-InP clad layer 1004 and thep-type contact layer 1005 is set to 1.0 μm.

By setting the thicknesses of the layers as described above, thei-InGaAsP core layer 1012 of the waveguide unit 1011 can be connected toa side face of the n-InGaAsP semiconductor layer 1002 of thephoto-detector unit 1001. The i-InGaAsP core layer 1012 of the waveguideunit 1011 and the n-InGaAsP semiconductor layer 1002 of thephoto-detector unit 1001 are connected adjacent to each other along thedirection in which the i-InGaAsP core layer 1012 extends.

In the waveguide unit 1011, for example, the width of the mesa structureincluding the i-InGaAsP core layer 1012 and the i-InP clad layer 1013(the width along the direction orthogonal to the direction in which thei-InGaAsP core layer 1012 extends) is 2.5 μm. In the photo-detector unit1001, the width of the mesa structure including the i-InGaAs absorbinglayer 1003, the p-InP clad layer 1004, and the p-type contact layer 1005(the width along the direction orthogonal to the direction in which thei-InGaAsP core layer 1012 extends) is 5 μm, and the length (PD length)of the photo-detector unit 1001 is 10 μm.

The light receiving element 1000 having the above-mentioned structurecan supply a detection signal with sufficient signal level to thesubsequent electrical circuit also at high frequencies while improvingthe efficiency of light absorption in the photo-detector unit 1001.Further, the light receiving element 1000 can perform an outputoperation adapted to high intensity light input in which the intensityof the input signal light is high.

In the above-mentioned embodiment, as an example of the light receivingelement 1000 with respect to a signal light having a wavelength of about1.5 μm, a photodiode in which the i-InGaAs absorbing layer 1003 is madeof InGaAs and layers such as a waveguide layer are made of anInGaAsP-based material is described. However, the embodiment is notlimited to this. In the light receiving element 1000 according to theabove-mentioned embodiment, the material of the i-InGaAs absorbing layer1003 may be another material that absorbs light within the wavelengthband of the incident signal light, and the material of other layers maybe another material that does not absorb this light.

While the material of layers such as the i-InGaAs absorbing layer 1003is an i-type semiconductor in the above example, for example, part orthe whole of the material of the InGaAs absorbing layer 1003 may be ap-type or n-type semiconductor.

While in the above-mentioned embodiment a high-mesa structure isdescribed as the waveguide structure in each of the waveguide unit 1011and the photo-detector unit 1001, the waveguide structure may be suchthat part or the whole of the structure is formed as a buried waveguide.

1-5. Manufacturing Method of the Light Receiving Element 1000

FIGS. 13A to 17B are diagrams each illustrating an example of themanufacturing process of the light receiving element 1000 illustrated asFIGS. 10 to 12B. Of FIGS. 13A to 17B, FIGS. 13A, 14A, 15A, 16A, and 17Aon the upper side are each a plan view as seen from above thecorresponding substrate, illustrating the main portion of the lightreceiving element 1000. FIGS. 13B, 14B, 15B, 16B, and 17B on the lowerside are cross-sectional views taken along broken lines XIIIB-XIIIB,XIVB-XIVB, XVB-XVB, XVIB-XVIB, and XVIIB-XVIIB in the plan views FIGS.13A, 14A, 15A, 16A, and 17A, respectively, illustrating the vicinity ofthe boundary region between the photo-detector unit 1001 and thewaveguide unit 1011. Hereinbelow, an example of the manufacturing methodof the light receiving element 1000 is described with reference to FIGS.13A to 17B.

As illustrated as FIGS. 13A and 13B, over an SI-InP substrate 1301, ann-InGaAsP film 1302, an i-InGaAs film 1303, a p-InP film 1304, and ap-InGaAs/InGaAsP laminate film 1305 made up of two films of p-InGaAs andInGaAsP are deposited by, for example, the metal organic chemical vapordeposition (MOCVD) method. At this time, the deposition is performed sothat the n-InGaAsP film 1302 has a thickness of 0.5 μm and the i-InGaAsfilm 1303 has a thickness of 0.5 μm. Also, the deposition is performedso that the total thickness of the p-InP film 1304 and thep-InGaAs/InGaAsP laminate film 1305 becomes 1.0 μm.

Next, a mask 1401 for covering a region that becomes the photo-detectorunit 1001 illustrated as FIGS. 10 to 12B is formed over thep-InGaAs/InGaAsP laminate film 1305, thereby selectively exposing aregion that becomes the waveguide unit 1011. As the mask 1401, forexample, a silicon oxide film is used. By etching according to relatedart using the mask 1401, as illustrated as FIGS. 14A and 14B, then-InGaAsP film 1302, the i-InGaAs film 1303, the p-InP film 1304, andthe p-InGaAs/InGaAsP laminate film 1305 are left only in thephoto-detector unit 1001, and these films are removed from the waveguideunit 1011. Through this processing, in the waveguide unit 1011, theSI-InP substrate 1301 is exposed.

Next, as illustrated as FIGS. 15A and 15B, over the SI-InP substrate1301 from which the waveguide unit 1011 is exposed, an i-InGaAsP film1501 and an i-InP film 1502 are deposited by selective growth accordingto related art using the MOCVD method. At this time, the deposition isperformed so that the i-InGaAsP film 1501 has a thickness of 0.5 μm andthe i-InP film 1502 has a thickness of 1.5 μm. Since the photo-detectorunit 1001 is covered with the mask 1401 used in the above-mentionedetching, growth of the i-InGaAsP film 1501 and the i-InP film 1502 inthe photo-detector unit 1001 can be inhibited. After depositing thei-InP film 1502, the mask 1401 is removed.

Next, a mask for covering a region that becomes a mesa structure in eachof the photo-detector unit 1001 and the waveguide unit 1011 is formedover the p-InGaAs/InGaAsP laminate film 1305 and the i-InP film 1502. Asthe mask, for example, a silicon oxide film is used. By etchingaccording to related art using this mask, a mesa structure is formed ineach of the photo-detector unit 1001 and the waveguide unit 1011. Afterthe etching, the mask is removed.

At this time, as illustrated as FIGS. 16A and 16B, in the photo-detectorunit 1001, the above-mentioned mask is used to remove thep-InGaAs/InGaAsP laminate film 1305, the p-InP film 1304, and thei-InGaAs film 1303, and the n-InGaAsP film 1302 is removed half waythrough its depth so as to leave part of the n-InGaAsP film 1302.Through this processing, part of the n-InGaAsP film 1302 is exposed.Thus, the mesa structure illustrated as FIGS. 10 to 12B is formed, whichincludes part of the n-InGaAsP semiconductor layer 1002, the i-InGaAsabsorbing layer 1003, the p-InP clad layer 1004, and the p-type contactlayer 1005.

As illustrated as FIGS. 16A and 16B, in the waveguide unit 1011, theabove-mentioned mask is used to remove the i-InP film 1502 and thei-InGaAsP film 1501, and part of the SI-InP substrate 1301 located belowthe i-InGaAsP film 1501 is also removed. Through this processing, partof the SI-InP substrate 1301 is exposed. Thus, the mesa structureillustrated as FIGS. 10 to 12B which includes the i-InGaAsP core layer1012 and the i-InP clad layer 1013 is formed.

Next, in the photo-detector unit 1001 and the waveguide unit 1011, thepassivation film 1017 made of a dielectric such as a silicon nitridefilm is formed, except for regions where electrodes are to be formed.Thereafter, as illustrated as FIGS. 17A and 17B, in the photo-detectorunit 1001, the p-side electrode 1015 is formed in a region at the top ofthe mesa structure where the p-type contact layer 1005 is exposed, by amethod according to related art such as metal deposition or plating.Also, the n-side electrode 1016 is formed in a region where then-InGaAsP semiconductor layer 1002 is exposed, by a method according torelated art such as metal deposition or plating. In the plan view ofFIG. 17A, the passivation film 1017 is not illustrated for the sake offacilitating understanding of the structure.

In FIGS. 17A and 17B, an electrode having an air bridge structure isused as the p-side electrode 1015. As is apparent from thecross-sectional view of FIG. 17B, due to this structure, the p-sideelectrode 1015, and the n-InGaAsP semiconductor layer 1002 to which then-side electrode 1016 is connected are electrically insulated by air.

Thus, parasitic capacitance occurring between the p-side electrode 1015and the n-side electrode 1016 can be reduced. Therefore, capacitance ofthe photo-detector unit 1001 can be made further smaller. Thus, thelight receiving element 1000 can supply a detection signal withsufficient signal level to the subsequent electrical circuit also atfurther higher frequencies.

However, the structure of the p-side electrode 1015 is not limited to anair bridge structure. An insulator may be formed in advance at thelocation where the p-side electrode 1015 is to be formed, and the p-sideelectrode 1015 may be formed over the insulator. In FIGS. 17A and 17B,the n-InGaAsP semiconductor layer 1002 partially remains in the portionof the PD opposite to the waveguide unit 1011, and the p-side electrode1015 is formed over the n-InGaAsP semiconductor layer 1002 via thepassivation film 1017. However, it is also possible to remove then-InGaAsP semiconductor layer 1002 in the portion of the PD opposite tothe waveguide unit 1011. This makes it possible to reduce thecapacitance of the p-side electrode 1015, and also further improvecharacteristics at high frequencies.

Although not illustrated in FIGS. 15A and 15B, in actuality, there arecases when the i-InGaAsP film 1501 is deposited with a small thicknessalso on the side wall portion of the photo-detector unit 1001 exposed byetching illustrated as FIGS. 14A and 14B. However, the film deposited onthe side wall portion of the photo-detector unit 1001 is sufficientlythin in comparison to the i-InGaAsP core layer 1012, and also therefractive index of the film deposited on the side wall portion is thesame as the refractive index of the i-InGaAsP core layer 1012. Thus,this film does not affect signal light propagation.

2. Second Embodiment

FIG. 18 illustrates an example of the structure of a light receivingelement 1800 according to a second embodiment. FIG. 18 illustrates across-section corresponding to the cross-section of the light receivingelement 600 according to the first embodiment illustrated as FIG. 7. Thelight receiving element 1800 illustrated as FIG. 18 differs from thelight receiving element 600 illustrated as FIG. 6 in the thickness ofthe core but is otherwise the same. Since a perspective view of thelight receiving element 1800 is the same as the perspective view of thelight receiving element 600 illustrated as FIG. 6 except for thethickness of the core, the perspective view is not illustrated.

As illustrated as FIG. 18, the light receiving element 1800 includes aphoto-detector unit 1801 provided over a substrate 1814, and a waveguideunit 1811 provided over the same substrate 1814.

The waveguide unit 1811 has a structure in which a core 1812 and anupper clad layer 1813 are laminated from the substrate 1814 side. Thematerial of each of the layers of this laminate structure is, forexample, a semiconductor. The waveguide unit 1811 has a mesa structureincluding the upper clad layer 1813 and the core 1812. A signal lightpropagates in the core 1812, and enters the photo-detector unit 1801.

The photo-detector unit 1801 has a structure in which an n-typesemiconductor layer 1802, an i-type absorbing layer 1803, a p-type upperclad layer 1804, and a p-type contact layer 1805 are laminated from thesubstrate 1814 side. The material of each of the layers of this laminatestructure is, for example, a semiconductor. The photo-detector unit 1801has a mesa structure including the p-type contact layer 1805, the p-typeupper clad layer 1804, the i-type absorbing layer 1803, and part of then-type semiconductor layer 1802. The n-type semiconductor layer 1802,the i-type absorbing layer 1803, and the p-type upper clad layer 1804form a PIN-type photodiode.

As illustrated as FIG. 18, in the light receiving element 1800, the core1812 is connected to a side face of the n-type semiconductor layer 1802,and the core 1812 and the n-type semiconductor layer 1802 are connectedadjacent to each other along the direction in which the core 1812extends. The core 1812 is so formed that its top face is located higherthan the bottom face of the n-type semiconductor layer 1802 (locatedfarther away from the substrate 1814), and its bottom face is locatedlower than the top face of the n-type semiconductor layer 1802 (locatedcloser to the substrate 1814).

The n-type semiconductor layer 1802 is so formed that its refractiveindex is higher than the refractive index of the core 1812, and lowerthan the refractive index of the i-type absorbing layer 1803. That is,the n-type semiconductor layer 1802 is so formed that its bandgapwavelength is longer than the bandgap wavelength of the core 1812, andshorter than the bandgap wavelength of the i-type absorbing layer 1803.The n-type semiconductor layer 1802 is formed so as to have acomposition that makes the absorption coefficient for the signal lightsufficiently small.

Further, as illustrated as FIG. 18, in the light receiving element 1800,the core 1812 is so formed that its top face is located lower than thetop face of the n-type semiconductor layer 1802 (located closer to thesubstrate 1814). The bottom face of the core 1812 is flush with thebottom face of the n-type semiconductor layer 1802. The structureillustrated as FIG. 18 can be formed by, for example, making thethickness of the core 1812 smaller than the thickness of the n-typesemiconductor layer 1802.

As illustrated as FIG. 18, in the light receiving element 1800, thesignal light propagates in the core 1812 in the waveguide unit 1811, andenters the n-type semiconductor layer 1802 in the photo-detector unit1801. The n-type semiconductor layer 1802 receives the signal light fromthe core 1812 along the direction in which the core 1812 extends. Sincethe refractive index of the n-type semiconductor layer 1802 is sethigher than the refractive index of the core 1812, the loss at the timewhen the signal light enters the n-type semiconductor layer 1802 fromthe core 1812 can be reduced. Part of the incident signal light seepsout from the n-type semiconductor layer 1802 into the i-type absorbinglayer 1803, and is absorbed in the i-type absorbing layer 1803.

In the photo-detector unit 1801, unlike in the photo-detector unit 101of the light receiving element 100 illustrated as FIGS. 1 and 2, thesignal light enters the n-type semiconductor layer 1802 which isdirectly in contact with the i-type absorbing layer 1803 in whichabsorption of the signal light takes place. Consequently, as soon as thesignal light enters the n-type semiconductor layer 1802, the signallight seeps out into the i-type absorbing layer 1803, and its absorptionbegins. Consequently, in the photo-detector unit 1801, the PD length canbe made shorter than the PD length of the photo-detector unit 101 in thelight receiving element 100 while ensuring sufficient light absorptionefficiency.

Because the PD length of the photo-detector unit 1801 can be shortened,capacitance of the photo-detector unit 1801 can be reduced.Consequently, the light receiving element 1800 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at high frequencies, making it possible for the subsequentelectrical circuit to process an input signal also at high frequencies.

In addition, in the light receiving element 1800, unlike in thephoto-detector unit 301 of the light receiving element 300 illustratedas FIGS. 3 and 4, the signal light does not directly enter the i-typeabsorbing layer 1803 but enters the n-type semiconductor layer 1802first, and then its components seeping out into the i-type absorbinglayer 1803 are absorbed. Consequently, in the photo-detector unit 1801,the overall density distribution of generated photocarriers is a flat,gently sloping distribution in comparison to that in the photo-detectorunit 301.

Consequently, in the photo-detector unit 1801, an excessive increase inlocal photocarrier density can be reduced even in the case of highintensity light input where the intensity of the inputted signal lightis high. Thus, it is possible to reduce deterioration of high frequencyproperty for high intensity signal light.

Further, in the light receiving element 1800, unlike in the lightreceiving element 600 illustrated as FIG. 7, the top face of the core1812 is located lower than the top face of the n-type semiconductorlayer 1802. Consequently, in the light receiving element 1800, ascompared with the light receiving element 600, seeping out of the signallight into the i-type absorbing layer 1803 becomes smaller by an amountof the difference between the top face of the core 1812 and the top faceof the n-type semiconductor layer 1802. That is, absorption per unitlength in the i-type absorbing layer 1803 becomes smaller than in thecase of the light receiving element 600, and the propagation distance ofthe signal light over which sufficient absorption takes place becomeslonger than in the case of the light receiving element 600.

Consequently, the peak of the photocarrier density distribution in thephoto-detector unit 1801 of the light receiving element 1800 occurs at alocation farther away from the end of the photo-detector unit 1801 fromwhich the signal light enters, than the peak in the case of thephoto-detector unit 601. Accordingly, in the light receiving element1800, the overall density distribution of generated photocarriersspreads to a location farther away from the end of the photo-detectorunit 1801, and thus becomes a more flat, gently sloping distribution ascompared with the distribution in the case of the light receivingelement 600. The value of density at the peak location of thephotocarrier density distribution becomes small in comparison to thevalue of density at the peak location in the case of the light receivingelement 600.

Accordingly, as compared with the photo-detector unit 601, in thephoto-detector unit 1801 of the light receiving element 1800, anexcessive increase in local photocarrier density can be reduced morealso when a signal light with higher intensity is inputted.Consequently, in the photo-detector unit 1801, deterioration of highfrequency property for high intensity signal light can be reduced. Thus,in the light receiving element 1800, it is possible to perform an outputoperation adapted to input of a signal light with higher intensity, incomparison to the light receiving element 600.

As has been described above, in the light receiving element 1800according to the second embodiment, a detection signal with sufficientsignal level can be supplied to the subsequent electrical circuit alsoat high frequencies while improving the efficiency of light absorptionin the photo-detector unit 1801. Further, the light receiving element1800 can perform an output operation adapted to high intensity lightinput in which the intensity of the input signal light is high.

Further, as compared with the light receiving element 600 according tothe first embodiment, the light receiving element 1800 according to thesecond embodiment can be also adapted to input of a signal light withhigher intensity. For example, the light receiving element 1800according to the second embodiment is effectively applied to a lightreceiving element, a light receiving device, and a light receivingmodule that process a signal light with higher intensity, and a systemusing these light receiving element, light receiving device, and lightreceiving module.

As a specific example of the structure of the light receiving element1800 according to the second embodiment, the configuration describedabove as a specific example of the structure of the light receivingelement 600 according to the first embodiment can be used. However, asmentioned above, the core 1812 is so formed that its thickness issmaller than the thickness of the n-type semiconductor layer 1802.

As for the manufacturing method of the light receiving element 1800 aswell, the method described above as the manufacturing method of thelight receiving element 600 can be used.

3. Third Embodiment 3-1. Structure of a Light Receiving Element 1900

FIG. 19 illustrates an example of the structure of a light receivingelement 1900 according to a third embodiment. FIG. 19 illustrates across-section corresponding to the cross-section of the light receivingelement 600 according to the first embodiment illustrated as FIG. 7. Thelight receiving element 1900 illustrated as FIG. 19 differs from thelight receiving element 600 illustrated as FIG. 6 in the thickness ofthe core but is otherwise the same. Since a perspective view of thelight receiving element 1900 is the same as the perspective view of thelight receiving element 600 illustrated as FIG. 6 except for thethickness of the core, the perspective view is not illustrated.

As illustrated as FIG. 19, the light receiving element 1900 includes aphoto-detector unit 1901 provided over a substrate 1914, and a waveguideunit 1911 provided over the same substrate 1914.

The waveguide unit 1911 has a structure in which a core 1912 and anupper clad layer 1913 are laminated from the substrate 1914 side. Thematerial of each of the layers of this laminate structure is, forexample, a semiconductor. The waveguide unit 1911 has a mesa structureincluding the upper clad layer 1913 and the core 1912. A signal light ismade to propagate in the core 1912, and enter the photo-detector unit1901.

The photo-detector unit 1901 has a structure in which an n-typesemiconductor layer 1902, an i-type absorbing layer 1903, a p-type upperclad layer 1904, and a p-type contact layer 1905 are laminated from thesubstrate 1914 side. The material of each of the layers of this laminatestructure is, for example, a semiconductor. The photo-detector unit 1901has a mesa structure including the p-type contact layer 1905, the p-typeupper clad layer 1904, the i-type absorbing layer 1903, and part of then-type semiconductor layer 1902. The n-type semiconductor layer 1902,the i-type absorbing layer 1903, and the p-type upper clad layer 1904form a PIN-type photodiode.

In the light receiving element 1900 illustrated as FIG. 19, the core1912 is connected to side faces of the n-type semiconductor layer 1902and i-type absorbing layer 1903. The core 1912, and the n-typesemiconductor layer 1902 and the i-type absorbing layer 1903 areconnected adjacent to each other along the direction in which the core1912 extends. The core 1912 is so formed that its top face is locatedhigher than the bottom face of the n-type semiconductor layer 1902(located farther away from the substrate 1914), and its bottom face islocated lower than the top face of the n-type semiconductor layer 1902(located closer to the substrate 1914).

The n-type semiconductor layer 1902 is so formed that its refractiveindex is higher than the refractive index of the core 1912, and lowerthan the refractive index of the i-type absorbing layer 1903. That is,the n-type semiconductor layer 1902 is so formed that its bandgapwavelength is longer than the bandgap wavelength of the core 1912, andshorter than the bandgap wavelength of the i-type absorbing layer 1903.The n-type semiconductor layer 1902 is formed so as to have acomposition that makes the absorption coefficient for the signal lightsufficiently small.

Further, as illustrated as FIG. 19, in the light receiving element 1900,the core 1912 is so formed that its top face is located higher than thetop face of the n-type semiconductor layer 1902 (located farther awayfrom the substrate 1914), and lower than the top face of the i-typeabsorbing layer 1903 (located closer to the substrate 1914). The bottomface of the core 1912 is flush with the bottom face of the n-typesemiconductor layer 1902. The structure illustrated as FIG. 19 can beformed by, for example, making the thickness of the core 1912 largerthan the thickness of the n-type semiconductor layer 1902.

However, it is preferable that the thickness of the n-type semiconductorlayer 1902 be not less than half the thickness of the core 1912. Thatis, it is preferable that the thickness of the portion of the core 1912connected to the i-type absorbing layer 1903 be not more than half theentire thickness of the core 1912. Also, it is preferable that half ormore portion of the core 1912 with respect to the thickness direction belocated lower than the top face of the n-type semiconductor layer 1902(located closer to the substrate 1914). Also, it is preferable that thethickness of the core 1912 be smaller than the sum of the thickness ofthe i-type absorbing layer 1903 and the thickness of the n-typesemiconductor layer 1902. This is to ensure that as compared with directentry of the signal light into the i-type absorbing layer 1903, lightabsorption in the photo-detector unit 1901 is performed substantiallyequally or predominantly by seeping out of the signal light from then-type semiconductor layer 1902 into the i-type absorbing layer 1903, sothat the value of photocarrier density near the end of thephoto-detector unit 1901 does not become excessively large. Details inthis regard are given later.

As illustrated as FIG. 19, in the light receiving element 1900, thesignal light propagates in the core 1912 in the waveguide unit 1911, andin the photo-detector unit 1901, the majority of the signal light entersthe n-type semiconductor layer 1902, and the remainder of the signallight directly enters the i-type absorbing layer 1903. The n-typesemiconductor layer 1902 and the i-type absorbing layer 1903 receive thesignal light from the core 1912 along the direction in which the core1912 extends. Since the refractive indexes of the n-type semiconductorlayer 1902 and i-type absorbing layer 1903 are set higher than therefractive index of the core 1912, the loss at the time when the signallight enters the n-type semiconductor layer 1902 and the i-typeabsorbing layer 1903 from the core 1912 can be reduced. Part of thesignal light having entered the n-type semiconductor layer 1902 seepsout from the n-type semiconductor layer 1902 into the i-type absorbinglayer 1903, and is absorbed in the i-type absorbing layer 1903. Thesignal light having entered the i-type absorbing layer 1903 is absorbedas it is in the region near the end of the i-type absorbing layer 1903through which the signal light enters.

In the photo-detector unit 1901, unlike in the photo-detector unit 101of the light receiving element 100 illustrated as FIGS. 1 and 2, themajority of the signal light enters the n-type semiconductor layer 1902which is directly in contact with the i-type absorbing layer 1903 inwhich absorption of the signal light takes place. Consequently, as soonas the signal light enters the n-type semiconductor layer 1902, themajority of the signal light seeps out into the i-type absorbing layer1903, and absorption of the signal light begins immediately after theentry. Consequently, in the photo-detector unit 1901, the PD length canbe made shorter than the PD length of the photo-detector unit 101 in thelight receiving element 100 while ensuring sufficient light absorptionefficiency.

Because the PD length of the photo-detector unit 1901 can be shortened,capacitance of the photo-detector unit 1901 can be reduced.Consequently, the light receiving element 1900 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at high frequencies. This makes it possible for the subsequentelectrical circuit to process an input signal also at high frequencies.

In addition, in the light receiving element 1900, unlike in the lightreceiving element 600 illustrated as FIG. 7, part of the signal lightdirectly enters the i-type absorbing layer 1903. Since absorption of theincident signal light takes place in the region near the end of thei-type absorbing layer 1903 from which the signal light enters, thevalue of photocarrier density near the end of the photo-detector unit1901 can be increased. Thus, in the light receiving element 1900, theefficiency of light absorption in the photo-detector unit 1901 can befurther improved as compared with the photo-detector unit 601.Accordingly, as compared with the light receiving element 600, the lightreceiving element 1900 can be also adapted to input of a signal lightwith lower intensity without increasing the PD length.

Also, in the light receiving element 1900, the further improvement inlight absorption efficiency makes it possible to further shorten the PDlength of the photo-detector unit 1901 as compared with the lightreceiving element 600. Further shortening of the PD length enables afurther reduction in capacitance of the photo-detector unit 1901.Consequently, the light receiving element 1900 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at further higher frequencies. This makes it possible for thesubsequent electrical circuit to process an input signal also at furtherhigher frequencies.//

On the other hand, in the light receiving element 1900, unlike in thephoto-detector unit 301 of the light receiving element 300 illustratedas FIG. 4, only part of the signal light directly enters the i-typeabsorbing layer 1903. Consequently, the value of photocarrier densitydoes not become excessively large near the end of the photo-detectorunit 1901 from which the signal light enters. Consequently, in thephoto-detector unit 1901, an excessive increase in local photocarrierdensity can be reduced even in the case of high intensity light inputwhere the intensity of the inputted signal light is high. Thus, in thelight receiving element 1900, it is possible to reduce deterioration ofhigh frequency property for high intensity signal light.

As has been described above, in the light receiving element 1900according to the third embodiment, a detection signal with sufficientsignal level can be supplied to the subsequent electrical circuit alsoat high frequencies while improving the efficiency of light absorptionin the photo-detector unit 1901. Further, the light receiving element1900 can perform an output operation adapted to high intensity lightinput in which the intensity of the input signal light is high.

Further, in the light receiving element 1900 according to the thirdembodiment, the efficiency of light absorption can be further improvedas compared with the light receiving element 600 according to the firstembodiment. Thus, the light receiving element 1900 can be also adaptedto detection of a signal light with lower intensity without increasingthe PD length. Also, the light receiving element 1900 can supply adetection signal with sufficient signal level to the subsequentelectrical circuit also at further higher frequencies.

For example, the light receiving element 1900 according to the thirdembodiment is effectively applied to a light receiving element, a lightreceiving device, and a light receiving module that process a signallight with lower intensity, and a system using these light receivingelement, light receiving device, and light receiving module. Also, thelight receiving element 1900 according to the third embodiment iseffectively applied to a light receiving element, a light receivingdevice, and a light receiving module for which an electrical circuitoperating at higher operating frequency is arranged in the subsequentstage, and a system using these light receiving element, light receivingdevice, and light receiving module.

As a specific example of the structure of the light receiving element1900 according to the third embodiment, the configuration describedabove as a specific example of the structure of the light receivingelement 600 according to the first embodiment can be used. However, asmentioned above, the core 1912 is so formed that its thickness is largerthan the thickness of the n-type semiconductor layer 1902.

As for the manufacturing method of the light receiving element 1900 aswell, the method described above as the manufacturing method of thelight receiving element 600 can be used.

3-2. Photocarrier Density Distribution in the Photo-Detector Unit 1901

FIG. 20 illustrates an example of the simulated density distribution ofphotocarriers generated in the photo-detector unit 1901 of the lightreceiving element 1900. The vertical axis represents the density ofphotocarriers as normalized on the basis of a predetermined value. Thehorizontal axis represents location inside the PD in the photo-detectorunit 1901.

In FIG. 20, the curves indicated by (d) to (f) represent photocarrierdensity distributions in the photo-detector unit 1901 of the lightreceiving element 1900 illustrated as FIG. 19. The distribution curveindicated by (d) represents the distribution in a case where thethickness of the portion of the core 1912 connected to the i-typeabsorbing layer 1903 is set to 25% of the entire thickness of the core1912. The distribution curve indicated by (e) represents thedistribution in a case where the thickness of the portion of the core1912 connected to the i-type absorbing layer 1903 is set to 50% of theentire thickness of the core 1912. The distribution curve indicated by(f) represents the distribution in a case where the thickness of theportion of the core 1912 connected to the i-type absorbing layer 1903 isset to 75% of the entire thickness of the core 1912.

In FIG. 20, for the sake of comparison, a photocarrier densitydistribution curve (b) for the photo-detector unit 301 of the lightreceiving element 300 illustrated as FIG. 3 is indicated by a brokenline, and a photocarrier density distribution curve (c) for thephoto-detector unit 601 of the light receiving element 600 illustratedas FIG. 6 is indicated by an alternate long and short dash line. Thephotocarrier density distribution curve (b) corresponds to a case wherethe thickness of the portion of the core 1912 connected to the i-typeabsorbing layer 1903 is set to 100% of the entire thickness of the core1912, and the photocarrier density distribution curve (c) corresponds toa case where the thickness of the portion of the core 1912 connected tothe i-type absorbing layer 1903 is set to 0% of the entire thickness ofthe core 1912.

[3-2-1. Comparison Between Cases Differing in the Thickness of thePortion of the Core 1912 Connected to the i-Type Absorbing Layer 1913]

Photocarrier density distribution in the photo-detector unit 1901 iscompared between cases that differ in the thickness of the portion ofthe core 1913 connected to the i-type absorbing layer 1913.

As represented by the distribution curve (d) in FIG. 20, in the casewhere the thickness of the portion of the core 1912 connected to thei-type absorbing layer 1903 is set to 25% of the entire thickness of thecore 1912, the peak of the photocarrier density distribution occurs at alocation separated by a predetermined distance from the end of thephoto-detector unit 1901 from which the signal light enters.

In the distribution curve (d), the value of density at the peak locationof the photocarrier density distribution is sufficiently small incomparison to the maximum value (the density near the end of thephoto-detector unit 1901) in the case of the distribution curve (b)(light receiving element 300). Also, the overall density distribution isa flat, gently sloping distribution as compared with the distributioncurve (b) (light receiving element 300).

That is, the distribution curve (d) has characteristics more similar tothe distribution curve (c) (light receiving element 600) than thedistribution curve (b) (light receiving element 300). Consequently, inthis case, it is considered that as in the case of the distributioncurve (c) (light receiving element 600), light absorption in thephoto-detector unit 1901 is performed predominantly by seeping out ofthe signal light from the n-type semiconductor layer 1902 into thei-type absorbing layer 1903, rather than by direct entry of the signallight into the i-type absorbing layer 1903.

On the other hand, as represented by the distribution curve (f) in FIG.20, in the case where the thickness of the portion of the core 1912connected to the i-type absorbing layer 1903 is set to 75% of the entirethickness of the core 1912, the maximum value of the photocarrierdensity distribution occurs at the end of the photo-detector unit 1901from which the signal light enters, and the photocarrier densitydistribution is concentrated near the end. Consequently, although lowerthan that of the distribution curve (b) (light receiving element 300),the photocarrier density is still high.

That is, the distribution curve (f) has characteristics more similar tothe distribution curve (b) (light receiving element 300) than thedistribution curve (c) (light receiving element 600). Consequently, inthis case, it is considered that as in the case of the distributioncurve (b) (light receiving element 300), light absorption in thephoto-detector unit 1901 is performed predominantly by direct entry ofthe signal light into the i-type absorbing layer 1903, rather than byseeping out of the signal light from the n-type semiconductor layer 1902into the i-type absorbing layer 1903.

In contrast, as represented by the distribution curve (e) in FIG. 20, inthe case where the thickness of the portion of the core 1912 connectedto the i-type absorbing layer 1903 is set to 50% of the entire thicknessof the core 1912, a small peak appears in the photocarrier densitydistribution at a location separated by a predetermined distance fromthe end of the photo-detector unit 1901, and a somewhat highphotocarrier density is observed in the region near the end of thephoto-detector unit 1901.

In the case of the distribution curve (e), unlike the distribution curve(b) (light receiving element 300) or the above-mentioned distributioncurve (f), the photocarrier density distribution has a relatively flatshape from the end of the photo-detector unit 1901. As a result, in thecase of the distribution curve (e), the maximum value of photocarrierdensity is lowered to roughly half the maximum value (the density nearthe end of the photo-detector unit 301) in the case of the distributioncurve (b) (light receiving element 300).

That is, the distribution curve (e) has characteristics roughlyintermediate between those of the distribution curve (c) (lightreceiving element 600) and distribution curve (b) (light receivingelement 300). Consequently, it is considered that in this case, theratio of contribution to light absorption in the photo-detector unit1902 is substantially equal between seeping out of the signal light fromthe n-type semiconductor layer 1902 into the i-type absorbing layer 1903and direct entry of the signal light into the i-type absorbing layer1903.

It is appreciated from the above that the thickness of the portion ofthe core 1912 connected to the i-type absorbing layer 1903 is preferablynot larger than half the entire thickness of the core 1912. This makesit possible to ensure that light absorption in the photo-detector unit1901 is performed substantially equally or predominantly by seeping outof the signal light from the n-type semiconductor layer 1902 into thei-type absorbing layer 1903, in comparison to direct entry of the signallight into the i-type absorbing layer 1903. Thus, an excessive increasein local photocarrier density can be reduced even in the case of highintensity light input where the intensity of the inputted signal lightis high.

[3-2-2. Comparison with the Light Receiving Element 600 (DensityDistribution (c))]

Next, photocarrier density distribution in the photo-detector unit 1901,601 is compared between the light receiving element 1900 and the lightreceiving element 600, with respect to a case where the thickness of theportion of the core 1912 connected to the i-type absorbing layer 1903 isset to 25% of the entire thickness of the core 1912 (densitydistribution (d) in FIG. 20).

In the case of the distribution curve (d) in FIG. 20, as compared withthe distribution curve (c) (light receiving element 600), the peak ofthe photocarrier density distribution is located closer to the end ofthe photo-detector unit 1901 from which the signal light enters.Accordingly, the overall photocarrier density distribution is alsoshifted to the side closer to the end of the photo-detector unit 1901,and the spread of the distribution as a whole is also small.

Thus, in the light receiving element 1900, the efficiency of lightabsorption can be further improved as compared with the light receivingelement 600. Thus, the light receiving element 1900 can be also adaptedto detection of a signal light with lower intensity without increasingthe PD length.

Also, in the light receiving element 1900, the further improvement inlight absorption efficiency makes it possible to further shorten the PDlength of the photo-detector unit 1901 as compared with the lightreceiving element 600. Further shortening of the PD length enables afurther reduction in capacitance of the photo-detector unit 1901.Consequently, the light receiving element 1900 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at further higher frequencies.

As has been described above, in the light receiving element 1900according to the third embodiment, the efficiency of light absorptioncan be further improved as compared with the light receiving element 600according to the first embodiment. Thus, the light receiving element1900 can be also adapted to detection of a signal light with lowerintensity without increasing the PD length. Also, the light receivingelement 1900 can supply a detection signal with sufficient signal levelto the subsequent electrical circuit also at further higher frequencies.

4. Fourth Embodiment

FIG. 21 illustrates an example of the structure of a light receivingelement 2100 according to a fourth embodiment. FIG. 21 illustrates across-section corresponding to the cross-section of the light receivingelement 600 according to the first embodiment illustrated as FIG. 7. Thelight receiving element 2100 illustrated as FIG. 21 differs from thelight receiving element 1900 illustrated as FIG. 19 in that a bufferlayer is formed between a substrate and a core, but is otherwise thesame. A perspective view of the light receiving element 2100 is notillustrated as in the case of the light receiving element 1900illustrated as FIG. 19.

As illustrated as FIG. 21, the light receiving element 2100 includes aphoto-detector unit 2101 provided over a substrate 2114, and a waveguideunit 2111 provided over the same substrate 2114.

The waveguide unit 2111 has a structure in which a buffer layer 2115, acore 2112, and an upper clad layer 2113 are laminated from the substrate2114 side. The material of each of the layers of this laminate structureis, for example, a semiconductor. The waveguide unit 2111 has a mesastructure including the buffer layer 2115, the upper clad layer 2113,and the core 2112. A signal light propagates in the core 2112, and enterthe photo-detector unit 2101.

As for the reason why the buffer layer 2115 is provided between thesubstrate 2114 and the core 2112, if the core 2112 is formed directly onthe surface of the substrate 2114, a situation can arise where whendepositing a film that becomes the core 2112 on the surface of thesubstrate 2114, a defect occurs within the film, making it difficult toobtain a sufficient film quality for the core 2112. The buffer layer2115 is provided to avoid such a situation.

As illustrated as FIG. 21, for reasons related to the manufacturingprocess, the buffer layer 2115 remains also in the side wall portion ofthe photo-detector unit 2101. That is, the buffer 2115 is inserted alsobetween an n-type semiconductor layer 2102 and an i-type absorbing layer2103. Generally, the buffer layer 2115 is formed of a material that hasa low refractive index in comparison to the core 2112. Consequently, asthe signal light having propagated through the core 2112 enters then-type semiconductor layer 2102 and the i-type absorbing layer 2103,loss of the signal light occurs in the buffer layer 2115. To reduce thisloss, it is preferable to form the buffer layer 2115 as thin aspossible.

The photo-detector unit 2101 has a structure in which the n-typesemiconductor layer 2102, the i-type absorbing layer 2103, a p-typeupper clad layer 2104, and a p-type contact layer 2105 are laminatedfrom the substrate 2114 side. The material of each of the layers of thislaminate structure is, for example, a semiconductor. The photo-detectorunit 2101 has a mesa structure including the p-type contact layer 2105,the p-type upper clad layer 2104, the i-type absorbing layer 2103, andpart of the n-type semiconductor layer 2102. The n-type semiconductorlayer 2102, the i-type absorbing layer 2103, and the p-type upper cladlayer 2104 form a PIN-type photodiode.

The core 2112 is connected to a side face of the n-type semiconductorlayer 2102 and a side face of the i-type absorbing layer 2103 via thebuffer layer 2115. The core 2112, and the n-type semiconductor layer2102 and the i-type absorbing layer 2103 are connected adjacent to eachother via the buffer layer 2115 along the direction in which the core2112 extends. The core 2112 is so formed that its top face is locatedhigher than the bottom face of the n-type semiconductor layer 2102(located farther away from the substrate 2114), and its bottom face islocated lower than the top face of the n-type semiconductor layer 2102(located closer to the substrate 2114).

In this specification, cases where the core is connected to a side faceof the n-type semiconductor layer or a side face of the i-type absorbinglayer also include cases where the core is connected to a side face ofthe n-type semiconductor layer or a side face of the i-type absorbinglayer via the buffer layer.

The n-type semiconductor layer 2102 is so formed that its refractiveindex is higher than the refractive index of the core 2112, and lowerthan the refractive index of the i-type absorbing layer 2103. That is,the n-type semiconductor layer 2102 is so formed that its bandgapwavelength is longer than the bandgap wavelength of the core 2112, andshorter than the bandgap wavelength of the i-type absorbing layer 2103.The n-type semiconductor layer 2102 is formed so as to have acomposition that makes the absorption coefficient for the signal lightsufficiently small.

Further, as illustrated as FIG. 21, the core 2112 is so formed that itstop face is located higher than the top face of the n-type semiconductorlayer 2102 (located farther away from the substrate 2114), and lowerthan the top face of the i-type absorbing layer 2103 (located closer tothe substrate 2114). The bottom face of the core 2112 is higher than thebottom face of the n-type semiconductor layer 2102 (located farther awayfrom the substrate 2114), and lower than the top face of the n-typesemiconductor layer 2102 (located closer to the substrate 2114). Thebottom face of the buffer layer 2115 is flush with the bottom face ofthe n-type semiconductor layer 2102. The structure illustrated as FIG.21 can be formed by, for example, making the thickness of the laminateincluding the core 2112 and the buffer layer 2115 larger than thethickness of the n-type semiconductor layer 2102.

However, it is preferable that the thickness of the n-type semiconductorlayer 2102 be not less than half the thickness of the core 2112. Thatis, it is preferable that the thickness of the portion of the core 2112connected to the i-type absorbing layer 2103 be not more than half theentire thickness of the core 2112. This is to ensure that lightabsorption in the photo-detector unit 2101 is performed predominantly byseeping out of the signal light from the n-type semiconductor layer 2102into the i-type absorbing layer 2103, rather than by direct entry of thesignal light into the i-type absorbing layer 2103, so that the value ofphotocarrier density near the end of the photo-detector unit 2101 doesnot become excessively large. Details in this regard are as describedabove with reference to the third embodiment.

As illustrated as FIG. 21, in the light receiving element 2100, thesignal light propagates in the core 2112 in the waveguide unit 2111, andin the photo-detector unit 2101, the majority of the signal light entersthe n-type semiconductor layer 2102, and the remainder of the signallight directly enters the i-type absorbing layer 2103. The n-typesemiconductor layer 2102 and the i-type absorbing layer 2103 receive thesignal light from the core 2112 along the direction in which the core2112 extends. Part of the signal light having entered the n-typesemiconductor layer 2102 seeps out from the n-type semiconductor layer2102 into the i-type absorbing layer 2103, and is absorbed in the i-typeabsorbing layer 2103. The signal light having directly entered thei-type absorbing layer 2103 is absorbed as it is in the region near theend of the i-type absorbing layer 2103 through which the signal lightenters.

In the photo-detector unit 2101, as in the photo-detector unit 1901 ofthe light receiving element 1900 illustrated as FIG. 19, majority of thesignal light enters the n-type semiconductor layer 2102 which isdirectly in contact with the i-type absorbing layer 2103 in whichabsorption of the signal light takes place. Consequently, in thephoto-detector unit 2101, as soon as the majority of the signal lightenters the n-type semiconductor layer 2102, the signal light seeps outinto the i-type absorbing layer 2103, and absorption of the signal lightbegins immediately after the entry. Consequently, in the photo-detectorunit 2101, the PD length can be made shorter than the PD length of thephoto-detector unit 101 in the light receiving element 100 whileensuring sufficient light absorption efficiency.

Because the PD length of the photo-detector unit 2101 can be shortened,capacitance of the photo-detector unit 2101 can be reduced.Consequently, the light receiving element 2100 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at high frequencies. This makes it possible for the subsequentelectrical circuit to process an input signal also at high frequencies.

In addition, in the light receiving element 2100, as in thephoto-detector unit 1901 of the light receiving element 1900, part ofthe signal light directly enters the i-type absorbing layer 2103. Sinceabsorption of the incident signal light takes place in the region nearthe end of the i-type absorbing layer 2103 from which the signal lightenters, the value of photocarrier density near the end of thephoto-detector unit 2101 can be increased. Thus, in the light receivingelement 2100, the efficiency of light absorption in the photo-detectorunit 2101 can be further improved as compared with the photo-detectorunit 601 in the light receiving element 600 illustrated as FIG. 7.Accordingly, as compared with the light receiving element 600, the lightreceiving element 2100 can be also adapted to input of a signal lightwith lower intensity without increasing the PD length.

Also, in the light receiving element 2100, the further improvement inlight absorption efficiency makes it possible to further shorten the PDlength of the photo-detector unit 2101 as compared with the lightreceiving element 600. Further shortening of the PD length enables afurther reduction in capacitance of the photo-detector unit 2101.Consequently, the light receiving element 2100 can supply a detectionsignal with sufficient signal level to the subsequent electrical circuitalso at further higher frequencies. This makes it possible for thesubsequent electrical circuit to process an input signal also at highfrequencies.

On the other hand, in the light receiving element 2100, unlike in thephoto-detector unit 301 of the light receiving element 300 illustratedas FIG. 4, only part of the signal light directly enters the i-typeabsorbing layer 2103. Consequently, the value of photocarrier densitydoes not become excessively large near the end of the photo-detectorunit 2101 from which the signal light enters. Consequently, in thephoto-detector unit 2101, an excessive increase in local photocarrierdensity can be reduced even in the case of high intensity light inputwhere the intensity of the inputted signal light is high. Thus, in thelight receiving element 2100, it is possible to reduce deterioration ofhigh frequency property for high intensity signal light.

As has been described above, in the light receiving element 2100according to the fourth embodiment, as in the case of the lightreceiving element 1900 according to the third embodiment, a detectionsignal with sufficient signal level can be supplied to the subsequentelectrical circuit also at high frequencies while improving theefficiency of light absorption in the photo-detector unit 2101. Further,the light receiving element 2100 can perform an output operation adaptedto high intensity light input in which the intensity of the input signallight is high.

Further, in the light receiving element 2100 according to the fourthembodiment, the efficiency of light absorption can be further improvedas compared with the light receiving element 600 according to the firstembodiment. Thus, the light receiving element 2100 can be also adaptedto detection of a signal light with lower intensity without increasingthe PD length. Also, the light receiving element 2100 can supply adetection signal with sufficient signal level to the subsequentelectrical circuit also at further higher frequencies.

As a specific example of the structure of the light receiving element2100 according to the fourth embodiment, as in the case of the lightreceiving element 1900 according to the third embodiment, theconfiguration described above as a specific example of the structure ofthe light receiving element 600 according to the first embodiment can beused. However, as the buffer layer 2115, for example, an InP layer witha thickness of 0.1 μm is formed. In addition, the laminate made up ofthe core 2112 and the buffer layer 2115 is so formed that its thicknessis larger than the thickness of the n-type semiconductor layer 2102.

As for the manufacturing method of the light receiving element 2100 aswell, as in the case of the light receiving element 1900 according tothe third embodiment, the method described above as the manufacturingmethod of the light receiving element 600 can be used. However, a stepof depositing an InP film at a thickness of 0.1 μm is added after theremoving step illustrated as FIGS. 14A and 14B, and before thedepositing step illustrated as FIGS. 15A and 15B.

5. Fifth Embodiment

FIG. 22 illustrates an example of the configuration of a light receivingdevice 2200 according to a fifth embodiment.

The light receiving device 2200 illustrated as FIG. 22 represents anexample of optical coherent receiver for demodulating a signal modulatedin qaudrature phase shift keying (QPSK) method. The light receivingdevice 2200 is promising from the viewpoints of miniaturization andassembly cost reduction as a waveguide-integrated type light receivingdevice in which an optical hybrid waveguide that convertsphase-modulated signal lights into intensity-modulated signal lights,and photodiodes (PDs) are integrated over the same substrate.

As illustrated as FIG. 22, the light receiving device 2200 includes aphoto-detector unit 2201, a connecting waveguide unit 2202, an opticalhybrid waveguide unit 2203, and an input waveguide unit 2204. Thephoto-detector unit 2201 includes four photodiode (PD) devices 2205 to2208. The connecting waveguide unit 2202 includes four connectingwaveguides 2209 to 2212. The optical hybrid waveguide unit 2203 includesa 90-degree optical hybrid waveguide 2213 having two inputs and fouroutputs. The input waveguide unit 2203 includes two input waveguides2214 and 2215.

The two input waveguides 2214 and 2215 of the input waveguide unit 2203are connected to the two inputs of the 90-degree optical hybridwaveguide 2213. The four connecting waveguides 2209 to 2212 of theconnecting waveguide unit 2202 are connected to the four outputs of the90-degree optical hybrid waveguide 2213. The four connecting waveguides2209 and 2212 are also connected to the corresponding PD devices 2205 to2208 of the photo-detector unit 2201.

In the light receiving device 2200, in the portions of the PD device2205 and the connecting waveguide 2209, the photo-detector unit 601,1801, 1901, or 2101 and the waveguide unit 611, 1811, 1911, or 2111 ineither one of the light receiving elements 600, 1800, 1900, and 2100according to the first to fourth embodiments illustrated as FIGS. 7, 18,19, and 21 can be used, respectively. The same applies to the portionsof the remaining PD devices and connecting waveguides, namely the PDdevice 2206 and the connecting waveguide 2212, the PD device 2207 andthe connecting waveguide 2210, and the PD device 2208 and the connectingwaveguide 2211. The optical hybrid waveguide unit 2203 and the inputwaveguide unit 2204 have the same layered structure as the connectingwaveguide unit 2202, and are formed over the same substrate as thephoto-detector unit 2201 and the connecting waveguide unit 2202.

Operation of the light receiving device 2200 is described. A QPSKmodulated signal light enters the input waveguide 2214, and a localoscillator (hereinafter referred to as LO) light enters the inputwaveguide 2215 as a reference light. The 90-degree optical hybridwaveguide 2213 receives the signal light and the LO light via the inputwaveguides 2214 and 2215, respectively. The 90-degree optical hybridwaveguide 2213 demodulates the QPSK modulated signal light by causinginterference between the LO light and the signal light, therebygenerating I-channel signal lights that are 180° out of phase with eachother and Q-channel signal lights that are 180° out of phase with eachother. The 90-degree optical hybrid waveguide 2213 outputs complementaryI-channel signal lights to the connecting waveguides 2209 and 2210, andoutputs complementary Q-channel signal lights to the connectingwaveguides 2211 and 2212.

The PD devices 2205 and 2206 receive the complementary I-channel signallights from the 90-degree optical hybrid waveguide 2213 via theconnecting waveguides 2209 and 2210, respectively. The PD devices 2205and 2206 each detect the received I-channel signal light as anelectrical signal, and generates an I-channel signal (electricalsignal). The PD devices 2207 and 2208 receive the complementaryQ-channel signal lights from the 90-degree optical hybrid waveguide 2213via the connecting waveguides 2211 and 2212, respectively. The PDdevices 2207 and 2208 each detect the received Q-channel signal light asan electrical signal, and generates an Q-channel signal (electricalsignal).

As the 90-degree optical hybrid waveguide 2213 described above, forexample, a 4×4 multi-mode interference (hereinafter referred to as MMI)waveguide having four inputs and four outputs can be used. In this case,the input waveguides 2214 and 2215 are each connected to two inputs ofthe 4×4 MMI waveguide. The connecting waveguides 2209 to 2212 areconnected to the four outputs of the 4×4 MMI waveguide.

In the light receiving device 2200 according to the fifth embodiment, asin the case of the light receiving elements 600, 1800, 1900, and 2100according to the first to fourth embodiments, the I-channel signals andthe Q-channel signals generated in the photo-detector unit 2201 can besupplied with sufficient signal level to the subsequent electricalcircuit also at high frequencies, while improving the efficiency oflight absorption in the photo-detector unit 2201.

The light receiving device 2200 can further perform an output operationadapted to high intensity light input in which the intensity of theinput signal light is high. Consequently, for example, when demodulatingphase-modulated signal lights by converting the signal lights intointensity-modulated signal lights (the complementary I-channel signallights and the complementary Q-channel signal lights) in the 90-degreeoptical hybrid waveguide 2213, even if the intensity of the LO light israised to increase the intensity of the converted signal lights, in thePD devices 2205 to 2208, deterioration of high frequency property forthe inputted high intensity signal light can be reduced.

In the four PD devices 2205 to 2208 included in the photo-detector unit2201, the corresponding n-type semiconductor layers are formedindependently. Thus, not only p-side electrodes but also n-sideelectrodes can be formed so as to be electrically isolated between theindividual PD devices 2205 to 2208, thereby ensuring sufficientelectrical isolation between the individual PD devices 2205 to 2208.Accordingly, unwanted crosstalk between the PD devices 2205 to 2208 canbe reduced, thereby making it possible for the light receiving device2200 to receive a signal light with little error.

While an optical coherence receiver is given as an example of the lightreceiving device 2200 of a waveguide-integrated type in the fifthembodiment mentioned above, the fifth embodiment is not limited to this.Any device in which PDs and waveguides are integrated may be used, andthe light receiving element 600, 1800, 1900, or 2100 according to eachof the first to fourth embodiments can be applied to such a device.

6. Sixth Embodiment

FIG. 23 illustrates an example of the configuration of a light receivingdevice 2300 according to a sixth embodiment.

A light receiving module 2300 illustrated as FIG. 23 represents anexample of optical coherent receiver module for demodulating a signalmodulated in dual polarization-quadrature phase shift keying (DP-QPSK)method.

As illustrated as FIG. 23, the light receiving module 2300 includesoptical coherent receivers 2301 and 2302, trans-impedance amplifiers(hereinafter referred to as TIA) 2303 to 2306, polarization lightsplitters (hereinafter referred to as PBS) 2307 and 2308, lenses 2309 to2314, and mirrors 2315 and 2316. Also, optical fiber cables 2317 and2318 are connected to the light receiving module 2300.

The light receiving module 2300 receives a DP-QPSK modulated signallight via the optical fiber cable 2317, and receives an LO light as areference light via the optical fiber cable 2318. The DP-QPSK modulatedsignal includes two signal lights having different, mutually orthogonalpolarization directions, and the two signal lights transmit signals thatare different from each other.

The DP-QPSK modulated signal light is made to enter the PBS 2307 via thelens 2309, and split by the PBS 2307 into two signal lights havingdifferent polarization directions. One of the split two signal lights ismade to enter the optical coherent receiver 2301 via the lens 2311, andthe other is made to enter the optical coherent receiver 2302 via themirror 2315 and the lens 2313. The LO light is similarly supplied toeach of the optical coherent receivers 2301 and 2302.

As each of the optical coherent receivers 2301 and 2302, the lightreceiving device 2200 according to the fifth embodiment illustrated asFIG. 22 can be used. The optical coherent receivers 2301 and 2302 eachreceive the QPSK modulated signal light and the LO light, anddemodulates the QPSK modulated signal by causing interference betweenthe LO light and the signal light.

The optical coherent receiver 2301 detects complimentary I-channelsignal lights obtained by the demodulation as complimentary electricalsignals (I-channel signals). The optical coherent receiver 2301 detectscomplimentary Q-channel signal lights obtained by the demodulation ascomplimentary electrical signals (Q-channel signals). The opticalcoherent receiver 2301 supplies the complementary I-channel signals(electrical signals) obtained by the detection to the TIA 2303, andsupplies the complementary Q-channel signals (electrical signals)obtained by the detection to the TIA 2304. Likewise, the opticalcoherent receiver 2302 supplies complementary I-channel signals to theTIA 2305, and supplies complementary Q-channel signals to the TIA 2306.

The TIAs 2303 to 2306 each receive the complementary I-channel signalsor the complementary Q-channel signals, and differentially amplifies thesignal level.

In the light receiving module 2300 according to the sixth embodiment, asin the light receiving device 2200 according to the fifth embodiment, itis possible to supply I-channel signals and Q-channel signals withsufficient signal level to each of the TIAs 2303 to 2306 also at highfrequencies, while improving the efficiency of light absorption in theoptical coherent receivers 2301 and 2302.

Further, the light receiving module 2300 can perform an output operationadapted to high intensity light input in which the intensity of theinput signal light is high. Consequently, for example, when demodulatingphase-modulated signal lights by converting the signal lights intointensity-modulated signal lights in each of the optical coherentreceivers 2301 and 2302, even if the intensity of the LO light is raisedto increase the intensity of the converted signal lights, deteriorationof high frequency property for the inputted high intensity signal lightcan be reduced.

In the plurality of PD devices included in each of the optical coherentreceivers 2301 and 2302, the corresponding n-type semiconductor layersare formed independently. Thus, not only p-side electrodes but alson-side electrodes can be formed so as to be electrically isolatedbetween the individual PD devices, thereby ensuring sufficientelectrical isolation between the individual PD devices. Accordingly,unwanted crosstalk between the PD devices can be reduced, thereby makingit possible for the light receiving module 2300 to receive a signallight with little error.

The light receiving element, the light receiving device, and the lightreceiving module according to each of the illustrative embodiments havebeen described above. However, the embodiments are not limited to theembodiments specifically discussed herein, and various changes andmodifications could be made to the embodiments without departing fromthe scope of the appended claims.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A light receiving device comprising: a firstwaveguide unit provided in a first region of a substrate, the firstwaveguide unit being configured to propagate a plurality of firstlights; a second waveguide unit provided in a second region of thesubstrate, the second waveguide unit being configured to receive theplurality of first lights and generate a plurality of second lights onthe basis of the plurality of first lights; a third waveguide unitprovided in a third region of the substrate, the third waveguide unitincluding a plurality of waveguides configured to propagate theplurality of second lights; and a photo-detector unit provided in afourth region of the substrate, the photodetector unit including aplurality of light receiving elements configured to receive theplurality of second lights from the plurality of waveguides, whereineach of the plurality of waveguides includes a core configured topropagate a corresponding one of the plurality of second lights, andwherein each of the plurality of light receiving elements includes: afirst semiconductor layer having a first conductivity type, the firstsemiconductor layer being configured to receive a corresponding one ofthe plurality of second lights from a corresponding one of the pluralityof cores along a first direction, the corresponding core extending inthe first direction, an absorbing layer configured to absorb thecorresponding second light received by the first semiconductor layer,and a second semiconductor layer having a second conductivity typeopposite to the first conductivity type.
 2. The light receiving deviceof claim 1, wherein the plurality of first lights include a signal lightand a reference light; and the second waveguide unit includes amulti-mode interference waveguide configured to generate the pluralityof second lights by causing interference between the signal light andthe reference light.